Thin film solar modules offer an attractive way to achieve low manufacturing cost with reasonable efficiency. These modules are made from a variety of materials, including amorphous silicon, amorphous silicon germanium, copper indium gallium selenide (CIGS), and cadmium telluride. A common feature of these solar modules is the deposition on a large area insulator such as a glass sheet.
Another common feature of these modules is the use of scribes and interconnects to divide the large area deposited layer into a number of cells and/or sub-cells. A top view of a typical module divided in this fashion is shown in FIG. 1. As shown in FIG. 1, a module 100 is divided into a plurality of cells 102 (i.e. stripes) that are series connected (e.g. electrically connected together in a horizontal direction in this drawing) via interconnects 104. The interconnects are typically formed in the module using scribes and conductors. However, it should be noted here that the length L of such modules 100 can be 1 meter or more. Meanwhile, the width of the interconnects (corresponding to the dimension W in FIG. 2), which typically run almost the entire length L of the module, are typically around 700-1000 μm, and the width of the cells (i.e. stripes) are typically about 1 cm. As will be understood by those of skill in the art, FIG. 1 is a simplified, not-to-scale drawing of a typical module, and that the module can further include other passive and active components not shown in FIG. 1 such as electrodes, protect diodes and terminals. Moreover, the module will typically also include external contacts and/or be environmentally encapsulated.
As is known, interconnects 104 are made to provide a high voltage, low current output that is less susceptible to series resistance losses. For example, a 1 m2 panel at 12% efficiency would provide 120 watts of power. If the cell operating voltage is 0.6 volts, then the current is 200 amps. Since the ohmic loss is I2R (where I is the current and R the resistance), and since the thin conductive films have relatively high resistance, most of the power would be dissipated. However, if the module was divided into 300 stripes, for example, then the voltage would be 180 volts and the current 0.56 amps. The ohmic losses would be reduced by a factor of 89,000.
Co-pending application Ser. No. 11/245,620, commonly owned by the present assignee, the contents of which are incorporated herein by reference, dramatically advanced the state of the art of forming interconnects for thin-film photovoltaic modules. One aspect of that invention included the use of a single laser scribe to form a cut that included a step structure to expose the base electrode. Another aspect of that invention was that the resulting interconnects could be much narrower than conventional interconnects, leading to more efficient module structures.
A process described in the co-pending application is shown in FIGS. 2A to 2E with reference to a portion of one interconnect region such as 106 in FIG. 1. In the first step shown in FIG. 2A, the entire conductor, semiconductor and contact stack 202-206 is deposited on the substrate 208, such as glass. In one embodiment, layer 202 is a metal such as molybdenum or a TCO such as ZnO, layer 204 is a semiconductor such as CIGS, and layer 206 is a TCO such as ZnO. In some embodiments, the entire stack is about 2-3 μm thick.
In the next step shown in FIG. 2B, a scribe 210 is made to the bottom conductor 202. As shown in FIG. 2C, a second scribe 212 is made using a smaller cut to create an exposed conductive ledge 214. Both of these scribes 210 and 212 may be made using a laser or mechanical scribe, or a combination of both.
In one embodiment where the scribes are made at the same time, a laser beam is used that has a skewed intensity profile, in that it is more intense on the left side than the right (with respect to the orientation of the drawing). This causes the left side to cut deeper than the right, forming the ledge 214. In another embodiment, two laser sources are coupled into a single fiber. One is an infrared source such as Nd:YAG with a wavelength of 1064 nm, for example, that penetrates the stack because its photon energy is below the bandgap of the semiconductor. This preferentially cuts through the conductor 202. The second is a shorter wavelength source, for example doubled Nd:YAG and 532 nm that cuts through the semiconductor 204 (e.g. CIGS) but not conductor 202. The width of the second cut is on the order of 20 to 50 μm, and the total width W can be reduced to as low as 0.01 to 0.2 cm, much narrower than was previously possible.
As shown in FIG. 2D, following the scribes, an insulator 216 is deposited on one wall. In a preferred embodiment, the insulator 216 is deposited using the following self-alignment method. A photosensitive polymer such as a polyimide or photoresist is applied over the entire module using any of a number of well-known methods, such an ink-jet, a spray or roller. The polymer is exposed from the back side through the glass. This performs a self-aligned exposure within the groove (i.e. the conductor layer 202 blocks exposure of all the photoresist except the portion in the groove). Next the polymer is developed, leaving only a coating on the left wall (with respect to the orientation shown in the drawing) that was exposed through the groove.
Finally, as shown in FIG. 2E, a conductor 218 is deposited over the insulator 216 to connect the top of the left cell 220 to the bottom of right cell 222. This provides a series connection between the cells 220 and 222. The entire length of the cut (e.g. the length L of the cut in the module as shown in FIG. 1) can then be coated with insulator and conductor materials to form the interconnects.
While the method of the co-pending application provides acceptable results and much narrower interconnects than previously possible, it may suffer from certain drawbacks. For example, laser ablation as used in the steps discussed in connection with FIGS. 2B and 2C has poor selectivity, so there can exist a narrow process window in which the ablation stops at the underlying conductor on the right side while cutting through to glass on the left. Moreover, laser ablation can cause damage at the edge, especially when performed at high ablation rates.
These drawbacks are especially apparent in connection with the laser cuts that form the conductive step. Therefore, it would desirable to overcome many of these shortcomings, particularly in connection with forming the conductive step, while not adding extra process complexity such as alignment requirements. The present invention aims at doing this, among other things.